« PreviousContinue »
Conodonts Aid Oil and Gas Investigations in Western United States
Conodonts are the microscopic (generally 0.004 to 0.039 inch in size) phosphatic hard parts of an extinct group of marine animals that are common to abundant in Cambrian through Triassic rocks (rocks deposited from 570 to 205 million years before present). Conodonts evolved and spread rapidly throughout most of their geologic range, and species distributions show a relation to paleogeography, particularly to seawater temperature and chemistry. Conodonts contain trace amounts of organic matter that undergo visible color changes from pale yellow to brown to black with increasing temperature as a result of a carbon-fixing process in the range of 60° to 300°C.; above 400°C., conodonts change from black to gray to opaque white and finally to crystal clear as a result of carbon loss, release of water of crystallization, and recrystallization. Thus, conodonts are useful chronologic, ecologic, and geothermal indicators.
Conodont research in the U.S. Geological Survey has led to the use of conodonts for determining several stratigraphic, structural, and organic-metamorphic aspects of oil and gas in the Western United States. The color changes in conodonts are used to create color-alteration maps that identify areas where past temperature conditions were favorable for generating and preserving oil and gas. In addition, conodont-based coloralteration maps for areas of proven oil and gas potential, such as the Western overthrust belt in Wyoming, can be used as comparative tools when evaluating similar color-alteration patterns found in other regions.
Figure 2A shows the main area of the U.S. Geological Survey conodont research in the Western United States. In Arizona, preliminary age-based conodont coloralteration (thermal assessment) maps were compiled. Figure 2B shows generalized areas of oil and gas potential in Paleozoic rocks (rocks deposited from 570 to 240 million years before present) that were derived from these maps; however, many local thermal highs related to extensive post-Paleozoic igneous activity are present in southeastern
Arizona that complicate the simple pattern shown. A preliminary conodont-alteration map for southwestern Montana (fig. 2C) shows a marked change in thermal maturity across the Medicine Lodge thrust indicating that this thrust may involve relatively large movement that placed more thermally mature (more deeply buried) western rocks above and adjacent to less thermally mature eastern rocks. This pattern is similar to that of the Absaroka thrust in the overthrust belt in Wyoming and suggests that both thrusts possibly formed at the same time and involve transport of similar magnitude. This similarity allows tenative correlation of these structures across the Snake River Plain in Idaho.
Conodont-based stratigraphic studies in southwestern Montana show that deposition of the Quadrant Sandstone within the thrust belt was initiated well before similar sandstone deposition on the craton to the east (approximately 10 million years earlier). This age difference, together with subtle changes in sedimentary facies, implies a late Paleozoic structural basin. The Quadrant Sandstone is a potential reservoir rock, and its distribution and depositional history, among other things, affect potential oil and gas preservation and migration.
The maps for assessing thermal maturity (conodont color-alteration index maps) in Ordovician through Triassic rocks in Nevada and Utah and adjacent parts of Idaho and California (Geological Survey Miscellaneous Investigation Map 1-1249) are being updated and expanded to include hundreds of new localities in these States and in parts of Arizona, Colorado, Idaho, Montana, New Mexico, and Wyoming and to serve as oil and gas exploration guides. In addition, an outcrop map of the Phosphoria Formation (a major potential Paleozoic petroleum source rock) throughout the overthrust belt in Wyoming and Idaho and the thrust belt in southwestern Montana has been compiled using conodont-based thermal-maturity assessments for preliminary oil and gas assessment.
Figure 2.- Conodont
research in the Western United States. A, Area of the Western United States being studied for conodontbased thermal assessment of Paleozoic-Triassic rocks; also shown are the locations of B and C. B, Map of Arizona showing generalized areas of relative oil and gas potential as assessed by conodont color alteration in Paleozoic rocks. C, Map of southwestern Montana showing Medicine Lodge thrust and eastern margin of the thrust belt; most Paleozoic and Triassic rocks east of the Medicine Lodge thrust are within the thermal window for oil and gas preservation.
Assessing the Nation's Hidden Mineral Resources
A dependable supply of strategic and critical minerals is basic to the economic vitality of our industrial society and to our national security. The United States is faced, however, with a continuing depletion of domestic mineral reserves and a greater dependence on importing a large number of mineral commodities from foreign sources. It is crucial that the Nation be able to respond effectively to adverse changes in international mineral supplies. An essential step in strengthening this response is the systematic mineral resource appraisal of the conterminous United States. Such action goes hand-in-hand with diplomatic, economic, and other measures needed to ensure a steady supply of minerals necessary to our national well being.
To properly assess the Nation's mineral resources, the U.S. Geological Survey initated the Conterminous United States Mineral Resource Assessment Program in 1977 as a companion program to the Alaska Mineral Resource Assessment Program, which was begun in 1974. This program is unique because it is the first Federal program structured to conduct systematic regional studies of the mineral resource potential of the conterminous United States. More than 66,000 square miles in 12 States have been appraised since the beginning of the program. During the next several decades, the program is scheduled to complete a mineral assessment of almost 1 million square miles, more than one-fourth the area of the entire United States, and will concentrate on areas that have a favorable potential for the occurrence of strategic and critical minerals.
A parallel thrust of the program is the development and application of new con
Future Conterminous United States Mineral Assessment Program Initiatives
cepts in the identification of significant mineral potential in heretofore untested, but possibly mineralized, areas. Traditional exploration methods used to identify and appraise mineral occurrences exposed at the surface are of limited value in those areas of the conterminous United States where overburden conceals a significant amount of bedrock which, in places, may be significantly mineralized. It is estimated that twothirds to three-quarters of the bedrock in the Western United States are concealed; even larger proportions of covered areas occur in other parts of the conterminous United States. An example is the midcontinent region where, until recent times, the only known mineral deposits were those outcropping at the surface. The potential for discovery of additional very large and economically significant mineral deposits in this region is excellent. At present, the State of Missouri alone produces 90 percent of the Nation's lead, 20 percent of its zinc, and minor, but significant, amounts of copper, silver, cobalt, nickel, and cadmium from vast stratabound deposits that are buried from 800 to 1,200 feet beneath the surface. However, this productive and promising region has received less than adequate attention because known exploration methods have offered little promise for discovery of concealed ore deposits existing in this environment except by inefficient and expensive “wildcat” drilling. It was for these reasons that the U.S. Geological Survey, in cooperation with the Missouri State Geological Survey, selected the Rolla quadrangle as one of the highest priority areas for the mineral assessment program investigations and began work in 1977.
Rolla and Springfield are 2 of 18 1:250,000-scale quadrangles that have been included in the Conterminous United States Mineral Assessment Program since its inception in 1977. The table shows the current status of these investigations.
The user base of the program's products is broad and diverse. An experimental public meeting was held in Salt Lake City, Utah, in December 1979 to present the significant preliminary results of program investigations in the Richfield 2° quadrangle. This meeting was so well received that five additional meetings have been held, and future meetings will be planned as investigations are completed. The published information on mineral resource potential will be used directly by decisionmakers for setting national mineral policy and by Federal, State, and local governments for land use planning, environmental impact analysis, and resource management activities. The basic geoscience and resource data on which the mineral assessment is based is being used by professional scientists in government, industry, and universities to make informed decisions regarding the availability of the Nation's mineral resources as compared to the Nation's mineral needs. These data are also used by the mineral industry in planning and developing their mineral exploration programs and by private organizations for evaluating their specific interests relative to the protection, conservation, and prudent utilization of the Nation's mineral wealth. The geologic, geochemical, and geophysical data generated by the program become a part of the reservoir of basic geoscience information that will be available to guide these critical decisions.
Applications of Space Shuttle Technology to Mineral Resource Appraisal
On November 12, 1982, the second flight of the Space Shuttle Columbia was launched, carrying the first scientific payload in its cargo bay. Included in the seven experiments was the Shuttle Multispectral Infrared Radiometer, which is a nonimaging instrument designed and built at the Calfornia Institute of Technology's Jet Propulsion Laboratory, Pasadena, California. This experiment is a cooperative project among the Jet Propulsion Laboratory, the U.S. Geological Survey, and the National Aeronautics and Space Administration.
The radiometer measures the spectrum of light reflected from areas on the ground that are 300 feet in diameter. The spectral reflectance thus recorded is divided by wavelength into 10 channels that are positioned so that certain minerals are conveniently identified. Five of these channels that are in the wavelength range of 2.0 to 2.4 micrometers (far beyond the light wavelengths visible to the human eye) have proved to be particularly valuable for appraising mineral resources using these remote methods.
Approximately 400,000 spectra were acquired by the infrared radiometer in the 1981 shuttle flights along 17 orbits under cloudfree conditions over the Eastern and Southern United States, Mexico, Southern Europe, North Africa, the Mid-East, and China, in spite of the reduction of the shuttle flight to 2.5 days from a planned 5-day flight. Storm systems over the Western United States prevented the radiometer from collecting data over most of that region, although it was a primary target area. Planned coverage of Australia, South America, and South Africa was precluded by unfavorable lighting conditions because of a 2-hour launch delay.
Selection of the 10 wavelength channels for the radiometer was based on analyses of more than 1,000 laboratory and field measurements of the spectral reflectance of rocks and soils. These analyses indicated that determination by remote methods of the mineral content of rocks could be improved substantially beyond that possible using Landsat data by making measurements in the 10 wavelength channels shown in figure 3. Landsat 1, 2, and 3 recorded reflectance in four channels located between
0.5 and 1.1 micrometers. The wavelength region between 2.0 and 2.5 micrometers is of particular interest because absorption of light by rocks and soils in this region allow identification of several minerals; some of these are associated with mineralogically altered rocks in potentially metallized areas. Although Landsat 4 measures reflectance in a broad channel in the 2.10- to 2.36-micrometer region, narrower wavelength channels are needed for mineral identification.
The importance of the five radiometer channels between 2.0 and 2.5 micrometers is illustrated by the two laboratory curves in figure 4 showing subtle, but significant, differences in brightness as the wavelength changes. In the curve representing calcite, the principal constituent in limestone, the brightness decreases to a minimum at 2.33 micrometers. The other curve (fig. 4) is typical of kaolinite, a clay mineral used in the ceramic industry and commonly associated with some metal deposits. The kaolinite spectrum is characterized by a brightness minimum centered near 2.20 micrometers and a less intense depression at 2.17 micrometers. The marked decreases in brightness in the calcite and kaolinite spectral curves are related to absorption of light by vibrations in the molecules. The wavelength position and intensity of the absorption features depend on the composition and molecular structure of the mineral and, hence, the analytical value of the spectral curves.
Each radiometer spectrum consists of brightness measurements made in the 10 channels for a 300-foot-diameter spot on the ground. The measurements were made at a rate of 128 individual 10-channel spectra per second and recorded on a digital tape recorder onboard the spacecraft. Photographs recorded during the operation of the radiometer are used for precisely locating the line of 300-foot spots on the ground.
Analysis of the data was initiated in southern Egypt because of the presence of excellent exposures of sedimentary rocks and a general lack of vegetation and rugged topography, except for a few steep escarpments. Figure 5 shows the radiometric data line superimposed on a generalized geologic map of the area. Eight spectra, each based on reflectance measurements made in the 10